The present invention relates to an improvement of an output structure of an X-ray image intensifier.
The X-ray image intensifier as one of image tubes has a structure substantially as shown in FIG. 1. Referring to FIG. 1, a vacuum envelope 11 comprises an input window 12 of an X-ray transmitting material and an output window 13 of a light-transmitting material. Within the envelope 11, an input screen 14 is disposed to oppose the input window 12, and an output screen 15 is disposed to oppose the output window 13. Furthermore, a focusing electrode 16 and an anode 17 are disposed in the envelope 11. Photoelectrons emitted from the input screen 14 are focused by an electron lens formed by the focusing electrode 16 and the anode 17, so that an optical image is formed on the output screen 15. The output structure having the output window 13 and the output screen 15 is illustrated in FIG. 2. Referring to FIG. 2, a glass substrate 20 is disposed opposite to the output window 13 of glass with a small gap 19. A phosphor layer 21 is formed on a surface of the glass substrate 20 which is located not opposing the output window 13. A light-reflecting film 22 made of a material such as aluminum is formed on the surface of the phosphor layer 21.
In the output structure of the X-ray image intensifier of this type, accelerated electrons 18 are incident on the phosphor layer 21, which then emits light. For example, when the phosphor particle located at a point 23 in FIG. 2 emits light, light 24 incident at an angle smaller than the critical angle passes through the output window 13 and is guided outside. However, light 25 incident at an angle greater than the critical angle is totally reflected by the surface of the glass substrate 20 which opposes the output window 13. The totally reflected light then returns to the phosphor layer 21 and is scattered from a point 26. The point 26, which differs from the light-emitting point 23, is brightened. As a result, the contrast and resolution of the output image are degraded.
Another output structure of the image tube which improves upon the above drawback is proposed in Japanese Patent Disclosure No. 53-24770 wherein a fiber plate is used in place of the glass substrate 20. In this output structure, the spatial frequency of image is limited by a size of fibers constituting the fiber plate. Furthermore, a decrease in fiber size is physically limited. A MTF (modulation transfer function) in a high spatial frequency region is decreased, thus resulting in a disadvantage. Another output structure using an A thin film of a thickness of several microns in place of the glass substrate 20 is proposed in Japanese Patent Disclosure No. 55-165558. However, the thin A film has a low mechanical strength and cannot serve as a substrate for the phosphor layer 21 in practice. When the thickness of the Al film exceeds one micron, electron transmission is difficult to perform.